DC--DC converters are normally used as constant DC voltage power supplies. The desirability of having a DC--DC converter that provides a number of different voltage outputs to a number of different independent loads has been recognized. There is a continuing need for such a circuit that is relatively simple in construction and relatively inexpensive, and which provides stable voltage levels to a number of independent loads that, if desired, can be electrically isolated. With electrical isolation, noise, impedance changes, and the like from one load does not affect the power transmitted to another load.
Multiple output DC--DC converters for providing a regulated voltage have typically required separate outputs, provided by separate transformer windings as well as independent direct voltage regulating controls for each output where precise regulation is required. Schottky diodes are typically used on low voltage outputs of DC--DC converters because of the low forward voltage drop. The forward voltage drop of a Schottky diode varies significantly with current and operating temperatures. Variation of the Schottky forward voltage drop is amplified by the converter transformer turns ratio and such variation produces voltage variation on the other unregulated outputs. The low voltage outputs require tight regulation due to the logic circuitry which the low voltage outputs support, and are therefore used to control the transformer duty cycle of the converter. It is therefore desirable to independently regulate the output of the other, unregulated outputs of the converter.
Previously approaches for output regulation have been to use a linear regulator which uses a pass element to regulate an unregulated output. However, this approach requires a high power dissipation in the pass element, which requires short circuit protection for the pass element.
A need has thus arisen for a circuit for regulating outputs of a multiple output DC--DC converter which is simple in operation and minimizes the number of circuit components.